Processes for the production of microporous films are well known in the art. For example, U.S. Pat. No. 3,870,593 (which is incorporated herein by reference) describes a process wherein a microporous film is produced by: (1) dispersing finely divided particles of a non-hygroscopic inorganic salt such as calcium carbonate in a polymer; (2) forming a film from the polymer; and (3) stretching the film to provide microporosity.
Methods of making composites of a microporous film and a nonwoven fabric are also known in the art. Such composites have been prepared by, for example, extrusion-coating a polymer film onto a nonwoven fabric. Prepared films and fabrics have also been bonded directly by a variety of means, including adhesive, thermal, and ultrasonic bonding.
It may also be desirable to stretch microporous film/fabric composites, however, stretching has its drawbacks. For instance, for microporous films, typical positive effects of stretching include higher vapor breathability and improved surface aesthetics. Vapor breathability (or water vapor transmission ratexe2x80x94xe2x80x9cWVTRxe2x80x9d) can be estimated by laboratory test methods, and is a function of the size and frequency of micropores in the film. Additional stretching of an already microporous film is known to increase the size of existing pores and create new pores. Therefore, highly stretched microporous films and microporous film/fabric composites generally have higher vapor breathability as compared to similar materials which have been stretched to a lesser degree.
Likewise, surface feel and drapability are known to be improved by stretching processes. Films and fabrics, when combined with one another, tend to be more stiff and harsh than either of the individual components alone. Stretching such composites tends to break down the rigid structure, thereby providing a softer surface feel and improved drapability.
On the other hand, stretching microporous film/fabric composites can result in decreased bond strength and increased pinholing. Stretching improves the softness and drapability by destroying the connection between film and fabric. This results in decreased bond strength in the laminate. Stretching can also cause undesirable damage to the laminate, such as pinholing, tearing, or shredding of the film, the fabric, or the composite as a whole.
The bonding of a film and fabric may be carefully controlled to avoid creating other functional and aesthetic problems. For example, in the case of extrusion coating a polyethylene film onto a spunbond polypropylene web, process conditions such as melt temperature and nip pressure determine the intrusion of the fibers into the film structure. At the maximum level of intrusion, the film and fabric essentially mold together and become one. Such a laminate, however, acquires the worst properties of the two components and tends to be both rigid and fragile. At the minimum level of intrusion, however, the film and fabric have little or no bond, and therefore tend to delaminate. Too much bond strength is also known to limit the amount of stretching which may be performed due to pinhole formation. Simply stated, if the bond between film and fabric is too large, the stretched film will sometimes fracture prior to delaminating, leaving a pinhole.
Rather than bonding a microporous film to a fabric, it is also possible to first bond a non-porous film to a fabric, and then stretch the resulting composite in order to render the film microporous. Previous attempts to first bond and then stretch film/fabric composites, such as that disclosed in U.S. Pat. No. 5,910,225 (which is incorporated herein by reference) have been only partially successful due to damage to the composite by the stretching process. Damage includes, but is not limited to, pinholes, tears, and other functional and aesthetic defects.
U.S. Pat. No. 6,066,221 describes a method of providing increased bonding between a film and nonwoven by applying lanes of hot air to the surface of laminates in the machine direction. Although this zoned hot air knife treatment increases the structural integrity of the laminate, stretching of the treated laminate results in debonding of the film and nonwoven.
U.S. Pat. No. 6,248,195 is another example of a bonding technique that is unable to prevent debonding of the film and nonwoven during post-stretching. Schmitz teaches that heated fluid or air can be used to bond webs at localized points forming a broken lane in the machine direction of the film.
U.S. Pat. No. 5,424,025 describes zone stretching of a film in the machine direction through the use of interpenetrating male and female rolls. Variations in the depth of engagement by the male roll creates an alternating pattern of heavy and lightly stretched sections.
U.S. Pat. No. 6,013,151 (which is incorporated herein by reference) teaches that a film/nonwoven fabric laminate can be made microporous and breathable upon incremental stretching at high speeds. The resulting microporous laminates have a high water vapor transmission rate (WVTR). It has been found that a flat film/nonwoven laminate can be incrementally stretched more uniformly than an embossed film/nonwoven laminate. More uniform stretching provides higher WVTR and fewer pinholes.
There is a continuing need for improvements in the performance and appearance of composites of polymer films and nonwoven fabrics. In particular, improvements are desired for producing microporous film/fabric composites with higher breathability, while avoiding pin holes and other functional and aesthetic defects.
The present invention relates to film/fabric composites and methods of producing the same which exhibit improved physical and aesthetic properties. A fabric structure is laminated to a film in a novel manner and then stretched so as to produce a breathable composite satisfactory for many end uses as a liquid barrier having high water vapor permeability. The film and fabric layers are bonded in lanes running in the machine direction. The composite then passes through a special stretching device designed so that all of the web except the highly bonded lanes are stretched. This invention is suitable for hygiene applications, such as producing diaper backsheets, which require composites that do not delaminate, provide high WVTR (water vapor transmission rate), and are soft and cloth-like.
This invention is able to utilize the positive aspects described above and avoid the negative after effects. The fabric is attached to the film with two levels of bond strength. The majority of the bond may be extremely weak, thereby allowing the majority of the composite to be stretched to the maximum degree, thus obtaining the desired high WVTR readings and aesthetic properties (e.g., surface softness and drapability). Since there are zones where the bond is high, the total material will not be subject to delamination. The areas where the bond is high are not stretched or are only partially stretched and, therefore, the product will not suffer the problem of pinholes which are caused by stretching areas tightly bonded.
The films and fabrics described here can be composited in many different ways including, but not limited to, extrusion coating, adhesive lamination, and thermal point bonding. The composite once formed is then subjected to stretching using a variety of techniques including, but not limited to, CD intermeshing ring rolls. The process of stretching highly-filled thermoplastic polymer films using techniques such as ring rolls is known in the art. One example of this method is described in U.S. Pat. No. 4,350,655.
The film/fabric composite may be made, for example, by first attaching a fabric to a film during production of that film. The fabric can be bonded to the film at the nip point in a cast operation via extrusion coating. Other methods of bonding before or after the cast station nip include hot melt adhesive and thermal or ultrasonic point bonding. Any of these three methods, as well as many other methods not mentioned but well known in the art, can be used in accordance with the process described herein. The only requirement on bonding technique is that the locations where the fabric is highly bonded to the film can be avoided during the stretching process.
One method that meets the above criteria is where increased bonding in certain regions of the composite is accomplished by a sonic sealer that thermally bonds fabric to film in lanes running in the machine direction. The sealer can be located anywhere after the cast station nip point. In either case, the increased bonding results in lanes where the fabric cannot be delaminated from the fabric with less than 150 grams per linear inch of peeling force. The other criteria of avoiding or reducing stretching activation can be accomplished by using CD intermeshing ring rolls with spaces where the bonding lanes fall.